The present invention relates to a method and apparatus for separating air to produce an oxygen product in which air is stripped within a distillation column operating at or near atmospheric pressure. More particularly, the present invention relates to such a method and apparatus in which the air is liquefied in a bottom reboiler of the distillation column. Even more particularly, the present invention relates to such a method and apparatus in which refrigeration is supplied by a liquid assist stream.
Air can be separated by a process known as cryogenic rectification to produce an oxygen product. One manner of conducting such a process is to separate compressed and purified air in a double distillation column unit. In such a distillation column unit, crude liquid oxygen is formed as a column bottoms of a higher pressure column. This crude liquid is further refined in a lower pressure column to produce the liquid oxygen product as a column bottoms. A product stream, withdrawn from the lower pressure column, is vaporized within a main heat exchanger that is initially used to cool the air. In order to produce the oxygen at pressure, the oxygen may be pumped prior to being vaporized.
As an alternative to a double distillation column unit, the compressed and purified air, after having been cooled, is introduced into an intermediate location of a single column. The oxygen forms as a bottoms product. This bottoms product is vaporized in a head condenser of the column. The resultant oxygen vapor may then be cold compressed. Part of the cold compressed vapor is returned to the column and a remaining part is warmed in the main heat exchanger to form the oxygen product.
In both of the double and single column arrangements, a sizable portion of the fabrication costs result from column construction. In case of double column plants, two columns are fabricated. In case of a single column plant, although there is a single column, such single column can more properly be visualized as a column having an additional lower section to separate the oxygen from the nitrogen. Additionally, in both types of systems, refrigeration is provided by machinery such as turbo expanders to maintain the plant in heat balance.
As will be discussed, the present invention provides a method and apparatus for producing an oxygen product in which the plant fabrication costs are less than those involved in fabricating both double and single column air separation plants of the prior art.